This invention relates to the preparation of soluble compounds containing hydroxyalkyl and urethane groups by alcoholysis from highly cross-linked insoluble distillation residues (from the commercial production of tolylene diisocyanate) which have reacted completely or almost completely with water. The products obtained as solutions, solvent-free melts or powders may be used as raw materials for the production of synthetic resins or their intermediates which are characterized by reduced combustibility. They may also be recyclized by simple hydrolysis into the tolylene diamines ("TDA") from which tolylene diisocyanate ("TDI") is derived.
The distillation of the phosgenation product of tolylene diamines results in high molecular weight, insoluble, secondary products containing uretdione, isocyanurate, carbodiimide, uretone imine, urea and biuret groups. Depending on the purity of the amines used as starting materials, the secondary products of the phosgenation also include methyl benzimidazolones which eventually react with the free isocyanate groups present to undergo an increase in molecular size by biuretization into insoluble, cross-linked products.
Partial recycling of these TDI distillation residues which are inevitably formed in enormous quantities in industrial processes has recently been possible by alkaline hydrolysis. However, only a relatively small proportion of tolylene diamines can be recovered by this extremely slow hydrolysis process, and it was only by a very laborous process that these tolylene diamines could be isolated in their pure form (U.S. Pat. Nos. 3,128,310 and 3,331,876).
Greater success in maximizing the yield of 2,4-TDI ("T 100") or of isomeric mixtures of 80% 2,4-TDI and 20% 2,6-TDI ("T 80") or 65% 2,4-TDI and 35% 2,6-TDI ("T 65"), based in each case on the quantity of tolylene diamine put into the process, has been achieved by using so-called "downpipe evaporators" as described in German Offenlegungsschrift No. 2,035,731. In these apparatus, reduced quantities of a sump phase which is almost free from monomers but still contains up to 25% by weight of free isocyanate groups is obtained. In order to prevent smoldering combustion which is liable to occur, as well as for physiological reasons, this phase is chilled with water to form a tar-like mass at about 150.degree. to 300.degree. C. immediately after removal of the pure monomeric TDI. In this denaturing process, the free isocyanate groups still present react to a large extent with water. The reaction is accompanied by vigorous evolution of carbon dioxide and the formation of additional urea and biuret groups so that only a small reside, in most cases about 1 to 10% by weight, of free isocyanate groups trapped by inclusion remains. In the text which follows, these cross-linked products which have been treated with water and are substantially free from monomers will be referred to as "denatured" TDI distillation residues. When the substance is stored in water or in a moist state, the isocyanate content gradually diminishes over a relatively long period.
The resulting denatured TDI residues which are obtained in the form of slags and contain polyureas, biuret, uretdione, carbodiimide and isocyanurate groups are completely insoluble in all the usual solvents. This is in contrast to the low molecular weight sump phase of distillation which still has a high isocyanate content before denaturation with water but is not stable in storage. Above 250.degree..+-.30.degree. C., these denatured TDI residues begin partially to melt, and decomposition occurs with evolution of gas. No technically or economically feasible solution has hitherto been found in the art for utilizing these residue slags, particularly the infusible TDI 80 residue slags which constitute by far the greatest proportion of slags obtained from TDI production and which have a very high molecular weight and are highly temperature resistant.
It was therefore an object of the present invention to produce conversion products from TDI residue slags which have been denatured with water to render them environmentally harmless, which conversion products would be utilizable in a homogeneous, liquid, form as raw materials for the production of synthetic resins with improved properties.
It has now surprisingly been found that liquid and soluble conversion products are obtained when completely insoluble highly cross-linked, slag-like, size-reduced tolylene diisocyanate distillation residues which have been denatured with water are reacted at elevated temperatures with low molecular weight compounds containing hydroxyl groups.
The low molecular weight and relatively high molecular weight secondary products obtained according to the invention which contain H-acidic groups may be used at any stage, even without a purification process and optionally through further intermediate stages, for the production of high quality synthetic resins with reduced combustibility. In special cases, they may simply be recycled to form the diamines originally put into the process.